Introduction to Micro-machining

The Micromachining is the process of machining very small parts with tools smaller than 0.015 inch in diameter and tolerances of just a few tenths. These kinds of micro holes are required in industries like micromachining is a versatile process and is used widely for machining plastic, glass, metal as well as preparing thin foils. The strength of this technique lies in the systematic use of `batch processing' and physical or chemical processes, with well-known advantages: Very low production costs; no theoretical limit to the possible miniaturization; very precise control of the material structure and composition; use of small quantities of material, which can be rare or very pure; the only possible method that can provide access to some physical phenomena.

Micro-electromechanical Systems (MEMS) based processes are mainly used in semiconductor industries such as for the fabrication of integrated circuits (IC), chips, etc. These processes require a tight work-room environment to precisely produce extremely small products. Most of these processes employ controlled chemical reaction for removal of material from workpiece. Till now the application of MEMS processes are confined mainly within electronic industries because of a large number of constraints and slow process. Few MEMS processes are enlisted below.

Surface micro-machining

LIGA (German acronym from Lithographie, Galvanoformung, Abforming translated to English as lithography, electroforming, and molding)

Whereas precision machining processes emphasize on surface generation, micro-machining processes mainly focus on micro-feature generation. Features of size down to 20µm can easily be fabricated by these processes. They can also produce quality surfaces; however, finishing capability is usually below that for precision machining processes. Examples of such processes include:

Micro-milling

Micro milling process has become an attractive method for the rapid prototyping of micro devices. The process is based on subtractive manufacturing method in which materials from a sample are removed selectively. A comprehensive review on the fabrication of circular and rectangular cross-section channels of microfluidic devices using micro milling process is provided this review work. Process and machining parameters such as micro-tools selection, spindle speed, depth of cut, feed rate and strategy for process optimization will be reviewed. A case study on the rapid fabrication of a rectangular cross section channel of a microflow cytometer device with 200 um channel width and 50 um channel depth using CNC micro milling process is provided. The experimental work has produced a low surface roughness micro channel of 20 nm in roughness and demonstrated a microflow cytometer device that can produce hydrodynamic focusing with a focusing width of about 60 um.

CASE STUDY Design of microfluidic

Since this study uses a micro milling a microfluidic design with a rectangular geometry will be used.  For example, microfluidics with 2 layer PMMA to be fabricated. The top layer has 4 holes with a diameter of 0.8 mm, the design of the hole is based on the need to place a tube with an outer diameter of 0.7 mm. While the design for the bottom layer of microfluidics, there is a circular inlet and outlet with a diameter of 0.6 mm which is smaller than the outer diameter of the tube, to allow the tube to be above the microfluidic layer and the entire fluid can enter the micro flow.

The tool used in this research is a 0.2 mm diameter tool made of carbide material, has 2 flutes and Aluminum coated. While the workpiece that will be used in this research is Poly (methyl methacrylate) or referred to as acrylic which has a thickness of 2 mm.

Micro-drilling

Drilling specialists generally use the term "micro drilling" to describe the drilling of holes smaller than 3 mm in diameter. This includes holes in the micron range, often encountered in the electronics industry, where many workpieces consist of wafer-thin material which is sometimes only a few hundredths or tenths of a millimeter thick. Holes this small are typically created by micro punching, micro laser cutting or micro EDM, though drilling tools can still be used for hole diameters as small as 30 µm.

More common, though, are the applications found at the upper end of the micro spectrum in general industry, the aerospace industry, mold and die making and medical equipment manufacturing. In these areas, drilled holes with a diameter of 2 to 3 mm are often required, sometimes at depths of up to 20xD or more. Examples include drilled holes for cooling, lubrication, venting or nozzle bores.

When it comes to successfully drilling very small diameter holes, an array of details and components are of crucial significance. It starts with the machine tool. Successful micro drilling requires a sensitive machine with a precise spindle and low runout. Not sure if the machine you’ve picked is suitable for the work? Have it evaluated by an expert in the field before beginning micro drilling operations. Similarly, the accuracy of your micro drilling operations will benefit from precision toolholders, typically hand clamped to ensure against positioning errors that can be introduced by an automatic toolchange system.

Microdrill Bits

Spade bits are the smallest variety of microdrill bits, and feature a cutting edge formed by two flat planes rather than a pointed tip. This cutting design, known as a “chisel edge,” removes material from a hole at a negative angle of extrusion. Its lack of a sharpened point means that the surface texture and slope of the workpiece can cause the drill bit to drift or enter the material at a slant, which must be compensated for with careful direction of the tool. Spade drills usually lack spiraled flutes, and this feature can result in smoother hole walls while also making debris removal more difficult.

Bits designed with a chisel edge are usually long in relation to their drilling diameter, creating a high level of thrust against the drilling axis. This means that as the cutting edges are expanding the diameter of the initial hole, the force along the chisel edge is lower than at the other segments of the bit. Microdrill bits are usually fabricated from a cobalt-steel alloy, which is relatively inexpensive, or tungsten carbide, which provides greater strength and durability.

Microdrill Spindles

Most microdrill spindles are designed in a vee-block arrangement, which is a common drilling configuration used for holding circular or curved workpieces. In microdrilling, a drill bit is attached to a holding piece, or mandrel, which is fastened between diamond bearing pads. The drill is designed to be concentric along an axis that aligns as closely as possible to the mandrel in order to reduce the chance of vibration. A drive belt runs from the drilling mandrel to an external motor, and exerts pressure that holds the mandrel against the diamond pads. In some cases, a sensor is included to gauge drilling force and bit wear.

The Microdrilling Cycle

Microdrills typically operate under the “peck cycle,” which periodically inserts and withdraws the drill bit from the hole to clear out accumulating debris. This is often accompanied with the application of cutting fluid, such as oil mist, that blows away larger chips from the hole. Not removing the debris, particularly the pieces larger than five micrometers, can lead to higher thrust force and greater axial pressure on the drill. However, because complete debris removal can cause a tapered hole to develop in softer materials, partial chip clearing is better suited for such workpieces.

Speed and Rotation Parameters

The speed and feeding rate for a microdrilling machine varies according to the project requirements, drill size, and workpiece material. When working with metals, average drill speeds are typically between 2000 and 4000 rotations per minute (rpm), while drilling plastics may require lower rates to reduce the risk of breakage or melting. Microdrilling often eschews higher speeds because allowing the drill to linger at the base of the hole under high rpm can lead to material hardening.

Micro-turning

Micro turning is an effective way to produce micro cylindrical or rotational symmetry components.  A micro part with a high aspect ratio can be achieved using the micro turning. The most serious problem encountered during micro turning is the cutting force which tends to bend the workpiece, and the machining force influences machining accuracy and the limit of machinable size. Micro turning is performed on either a conventional precision machine or a micro turning system.

Diamond turning of the micro structured surface can be regarded as another group of micro cutting. With the aid of fast tool servos (FTS), complex micro structured surfaces can be generated by diamond turning.

Advantages of micro-machining:

The main benefit of Micromachining using specialized techniques and tools is that it allows the reliably repeatable as efficient production of small and intricate parts that have tight tolerances.

Micromachining offers a method for single-process machining for smaller parts, so milling and turning can be done on the same machine. This reduces lead time and allows parts to be machined more efficiently.

Micromachining is ideal for machining prototypes and parts with micro features in both plastics and metals and has a variety of applications.

Micromachining using machines with high spindle speeds or Swiss-type lathes has the advantage of creating cleaner cuts, more precise dimensions, and tighter tolerances to fit their specialized applications in the semiconductor and medical industries.

Implementing Micromachining in your precision engineering practice provides the opportunity to take on a greater range of scope of bids and make more diverse and specialized parts. Even larger parts can be machined with greater accuracy and speed on machines used for micromachining.

 

Disadvantages of micro-machining:

• Higher fabrication steps involved and hence expensive.

• Complex technology involving process of repetitive deposition of thin films on wafer, photo patterning them and etching them.

• Difficult to implement for large structures